The California Highway Patrol says it happened just after 5 pm Sunday.
Investigators say the man lost control of his car after being shot and then crashed into the median.
The victim was found inside his car and taken to the hospital with life-threatening injuries.
Authorities are still trying to establish where exactly the shooting occurred, and no potential suspect information has been released at this time.
The crash and law enforcement response shut down the westbound and eastbound connectors to northbound Highway 99.